While ε-Fe2O3 is an extremely rare phase among iron oxides, particles thereof having a nanometer order size show a great coercive force (Hc) of approximately 20 kOe (1.59×106 A/m) at room temperature, and thus a production method for synthesizing ε-Fe2O3 as a single phase has been investigated (PTL 1). In the case where ε-Fe2O3 is used in a magnetic recording medium, there is no material in the current situation for a magnetic head having a high-level saturation magnetic flux density corresponding thereto, and thus the adjustment of the coercive force is performed by substituting a part of Fe sites of ε-Fe2O3 by a trivalent metal, such as Al, Ga, and In, and the relationship between the coercive force and the radio wave absorption characteristics is also investigated (PTL 2).
In the field of magnetic recording, development of a magnetic recording medium having a high carrier to noise ratio (C/N ratio) of the reproduced signal level and the particulate noise has been performed, and for increasing the recording density, the magnetic particles constituting the magnetic recording layer are demanded to be refined. However, the refinement of the magnetic particles generally tends to cause deterioration of the environmental stability and the thermal stability, which raises concerns about the deterioration of the magnetic characteristics of the magnetic particles under the use or storage environment, and accordingly, various partially substituted materials of ε-Fe2O3 represented by the general formula ε-AxByFe2-x-yO3 or ε-AxByCzFe2-x-y-zO3 (wherein A represents a divalent metal element, such as Co, Ni, Mn, and Zn; B represents a tetravalent metal atom, such as Ti; and C represents a trivalent metal element, such as In, Ga and Al) that have a reduced particle size and a variable coercive force and are excellent in environmental stability and thermal stability, by substituting a part of Fe sites of ε-Fe2O3 by another metal that is excellent in heat resistance have been developed (PTL 3).
ε-Fe2O3 is not a thermodynamically stable phase, and therefore the production thereof requires a special method. PTLs 1 to 3 shown above describe such a production method of ε-Fe2O3 that fine crystals of iron oxyhydroxide formed by a liquid phase method are used as a precursor, and the precursor is coated with a silicon oxide by a sol-gel method and then subjected to a heat treatment, and as the liquid phase method, a reverse micelle method using an organic solvent as the reaction medium, and a method using only an aqueous solution as the reaction medium are described respectively.